Electroacoustic transducer



Sept. 19, 1950 R. c. MINER ELECTROACQUSTIC TRANSDUCER Filed Oct. 19,1946' 05 o u a a 8 I U |m w m F m 2 2 W H iL N s I |1|. F

//v VEN TOR R. C. MINER A T TORNE K Patented Sept. 19, 1950ELEcTRoAcoUs'rrC I v H I 'R'us'se ll C. Miner, Chatham, N. J., assignorto Bell Telephone Laboratories, Incorporated, New York, N. Y.,facorporation of New York Application October 19,1946, ser al No. 704,377

This invention relates to electroa'coustic transducers and moreparticularly to magnetic structures especially suitable for ringarmature type transducers, such as disclosed in patent 2,249,160,granted July 15, 1941 to Edward Mott.

Ring armature type transducers comprise, in general, a magnetic systemincluding coaxial, inner and outer cylindrical pole members, an annulararmature supported at or adjacent its periphery upon the outer polemember and having its inner marginal portion in juxtaposition to anddefining an air gap with the inner pole member, and a diaphragm coupledto the armature. The magnetic system includes a permanent magnet which,in order to provide a high polarizing flux,

of similar performance characteristics for similar transducers requiresthat the air gaps in the several devices be of essentially the samelength.

In constructions wherein a dimension of the magnet is an importantparameter in the determination of the air gap between the armature andthe inner pole member, working of the magnet, for example grinding ofone face thereof, may be necessary. Such working, particularly ifgrinding to close tolerances is necessary, because of the character ofthe magnet material, is difficult and relatively expensive.

One general object of this invention is to simplify the construction ofmagnetic systems for electroacoustic transducers and thereby tofacilitate the manufacture thereof in quantity and at relatively lowcost.

More specifically, objects of this invention are to expedite theassembly of magnetic systems for electroacoustic transducers of the ringarmature type and to enable the ready and economic determination of aprescribed air gap of accurate length in such devices.

In accordance with one feature of this invention, the magnetic structureof a ring armature transducer is constituted in part by a pair of cupshaped pole pieces mounted one within the other and a magnet disposedbetween the pole pieces and in contact therewith. The pole pieces areformed of a relatively easily workable material, for example aniron-nickel alloy known as Permalloy, and the rims or edges thereof areground so "that they are accurately coplanar, whereby the edge of theouter pole piece provides a, reference surface for determining thenormal. length of the air gap between the armature and the inner polepiece. Thus,this air gap may be fixed accurately and readily withoutworking of the magnet; Further, the magnet may beof simple form, forexample a'rectangular or circular block, easily cast and, hence, ofrelatively low cost.

The invention and the above noted and other features thereof will beunderstood more clearly and fully'zfrom the following detaileddescription with reference to the accompanying drawing in which:

Fi 1 is apla'n view of a ring armature type electr'oac'ousti'ctransducer illustrative of one embodiment of this invention; and

. Fig; Zis a sectional view or the transducer taken along plane 2-2 ofFig. 1

Referring now to the drawing, the transducer therein illustrated, whichmay be utilized for either translation of sound into electrical signalsor vice versa, comprises a pair of cup shaped pole pieces, for exampleof Permalloy, mounted one within the other. The pole pieces comprisecoaxial cylindrical portions l0 and ii respectively and parallel baseportions 12 and I3 respectively.

Disposed between the base portions I2 and I3 and secured thereto, as bysoldering, is a block magnet l4', for example of Alnico. The magnet maybe circular as illustrated or of other form, e. g. rectangular, and ismagnetized in the direction of its thickness, as indicated in Fig. 2, sothat the two pole pieces are of opposite polarity.

In the fabrication of the device, after the pole pieces and magnet areassembled, the edges of the pole pieces, i. e., the upper faces or endsof th cylindrical portions It and II in Fig. 2, are groundsimultaneously so that they are accurately coplanar.

Seated upon the outer cylindrical portion II isan annular magneticspacer l5 of Permalloy for example, upon which the peripheral portion.of an annular armature l6, coaxial with the pole pieces and for exampleof Permalloy rests. Asshown clearly in Fig. 2, the inner marginalportion of the armature overlies the end face of the cylindrical portion10 of the inner pole piece- The normal length of the air gap between thearmature and the end face noted is fixed, as is apparent, by thethickness of the spacer I5.

The armature carries a dished non-magneticdiaphragm ii the periphery ofwhich is secured to the armature as by a suitable cement.

A non-magnetic spacer I8 is seated upon the spacer I5, extends over thearmature l6 and has seated thereon an annular, radially magnetizedmagnet l9. The magnet extends laterally over the pole piece portions Iand H and, as indicated in Fig. 2, is so magnetized that each of itspoles is of polarity opposite that of the pole piece face injuxtaposition thereto. Advantageously, the auxiliary magnet is of lessstrength than the main magnet whereby a partial unbalance in themagnetic circuit obtains at the armature.

Disposed between the spacer l and auxiliary magnet I9 is a magneticannulus 20, which provides a low reluctance path between the pole pieceportion H and the auxiliary magnet.

An annular signal coil 2| partly encased in an insulating jacket 22 ismounted between the cylindrical pole piece portions I0 and II. The baseportions 12 and I3 of the pole pieces may be provided with a pluralityof apertures 23 and 24 respectively, the former having thereover sheets25 of acoustic resistance material, to provide in combination with thechamber between the diaphragm l1 and inner pole piece an acousticnetwork of mass, stiffness and resistance for controlling vibration ofthe diaphragm, specifically to damp the response peak due to thediaphragm resonance and to enhance the response at other frequencies.

Reference is made of the application Serial No. 704,483 filed October19, 1946, of Edward E. Mott wherein certain features of the constructionherein described are disclosed and claimed.

Although a specific embodiment of this invention has been shown anddescribed, it will be understood that it is but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. An electroacoustic transducer comprising a first pole piece having abase and a cylindrical portion, a magnet seated upon said base andwithin said pole piece, a second pole piece within said first pole pieceand having a base seated upon said magnet and a cylindrical portioncoaxial with and spaced from said first cylindrical portion, said magnetmagnetizing said first and second pole pieces to opposite polarity, saidcylindrical portions having coplanar pole faces, a spacer member seatedupon the pole face of the cylindrical portion of said first pole piece,an annular armature coaxialwith said pole pieces, seated adjacent itsperiphery upon said spacer member and extending therefrom over the poleface of the cylindrical portion of said second pole piece, and adiaphragm member coupled to said armature.

2. An electroacoustic transducer comprising a one piece, inner cupshaped pole piece having a base and a flat circular pole face, a disheddiaphragm projecting into said pole piece and having its periphery injuxtaposition to said pole face, an outer one piece cup shaped polepiece having its base opposite the base of said inner pole piece andhaving also a flat circular pole face coaxial and coplanar with saidpole face of said inner pole piece, a thin magnetic washer seated uponsaid pole face of said outer pole piece, a fiat circular magneticarmature coaxial with said pole faces, seated adjacent its outer marginupon said washer and having its inner marginal part joined to theperiphery of said diaphragm, and a block magnet between and joined tothe bases of said inner and outer pole pieces.

RUSSELL C. MINER.

REFERENCES CITED The followingreferences are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,991,738 Gaubert Feb; 19, 19352,078,385 Kato Apr. 27, 1937 2,249,160 Mott July 15, 1941 2,395,166Collins Feb. 19, 1946 2,426,948 Preston Sept. 2, 1947 FOREIGN PATENTSNumber Country Date 506,480 Great Britain May 30, 1939

